The gas phase reaction of singlet dioxygen with water: A water-catalyzed mechanism

Stimulated by the recent surprising results from Wentworth et al. [Wentworth, A. D., Jones, L. H., Wentworth, P., Janda, K. D. & Lerner, R. A. (2000) Proc. Natl. Acad. Sci. USA 97, 10930-10935] that Abs efficiently catalyze the conversion of molecular singlet oxygen (O-1(2)) plus water to hydrogen peroxide (HOOH), we used quantum chemical methods (B3LYP density functional theory) to delineate the most plausible mechanisms for the observed efficient conversion of water to HOOH We find two reasonable pathways. In Pathway I, (i) H2O catalyzes the reaction of O-1(2). with a second water to form HOOOH; (ii) two HOOOH form a dimer, which rearranges to form the HOO-HOOO + H2O complex; (iii) HOO-HOOO rearranges to HOOH-OOO, which subsequently reacts with H2O to form H2O4 + HOOH; and (iv) H2O4 rearranges to the cyclic dimer (HO2)(2), which in turn forms HOOH plus O-1(2) or O-3(2). Pathway II differs in that step ii is replaced with the reaction between HOOOH and O-1(2), leading to the formation of HOO-HOOO. This then proceeds to similar products. For a system with O-18 H2O. Pathway I leads to a 2.2:1 ratio of O-16:O-18 in the product HOOK whereas Pathway II leads to 3:1. These ratios are in good agreement with the 2.2:1 ratio observed in isotope experiments by Wentworth et al. These mechanisms lead to two HOOH per initial O-1(2) or one, depending on whether the product of step iv is O-1(2) or O-3(2), in good agreement with the experimental result of 2.0. In addition to the Ab-induced reactions, the hydrogen polyoxides (H2O3 and H2O4) formed in these mechanisms and their decomposition product polyoxide radicals (HO2, HO3) may play a role in combustion, explosions, atmospheric chemistry, and the radiation chemistry in aqueous systems.